PRISTINA, Kosovo -- Peacekeepers said Serbian rioters in Kosovo "crossed a red line" as the world community urged calm in the most violent protests since Kosovo's independence.

French Lt. Gen. Xavier Bout de Marnhac with the NATO-led peacekeeping force in Kosovo, said Serbian rioters in the town of Mitrovica "crossed a red line with the deliberate intent to kill people," adding "we will not tolerate that any more," Duetsche Welle reported Wednesday.

Residents in the Serb-dominated town of Mitrovica responded to the capture of a U.N. court in a U.N police operation. Residents attacked the police, prompting NATO forces to respond. One U.N. peacekeeper died from his injuries Monday.

NATO placed the town under de facto military law Tuesday.

Though not the first incidents of violence since Kosovo declared independence in February, U.N officials said it was the worst such outbreak since then.

Larry Rossin with the U.N. Assistance Mission in Kosovo accused Serbians of organizing the violence. "We know who they are," Rossin warned.

U.N. Secretary-General Ban Ki-Moon urged "all communities to exercise calm and restraint" in a statement and U.S. Secretary of State Condoleezza Rice made similar statements from Moscow.